1. Field of the Invention
The present invention relates to a process for producing aragonite crystal form calcium carbonate with a large particle size and an acicular shape (Hereinafter referred to as "aragonite calcium carbonate").
2. Description of Prior Art
Aragonite crystal form calcium carbonate (hereinafter referred to as "aragonite" or "aragonite calcium carbonate") is superior in dispersibility of particles to calcite crystal form calcium carbonate (hereinafter referred to as "calcite" or "calcite calcium carbonate"). Furthermore, a cylindrical crystal habit of aragonite which does not exist in the calcite, shows various characteristics which are not expected in the calcite. Therefore, more effective industrial production of aragonite have been attempted up to now. But, aragonite prepared by each of those processes is almost fine acicular shape (nearly 0.5-3.0 .mu.m in a long diameter, 0.1-0.7 .mu.m in a short diameter) and have been utilized on the surface of coated paper to improve its printability. For example, as the prior art for gas-liquid reaction of Ca(OH).sub.2 slurry and CO.sub.2 gas, a process for regulating an amount of CO.sub.2 gas stepwise in the carbonation step is proposed (Japanese Patent Publication No. Sho 55-51852,) and a process for adding in advance a crystal nucleus formation agent into Ca (OH).sub.2 slurry is disclosed (Japanese Patent Laid Open No. Sho 59-223225).
On the one hand, a production of a larger acicular aragonite has been expected to develop the various industrial use. Although processes forming larger acicular-shaped aragonite are disclosed in Japanese Patent Laid Open No. Sho 62-278123, Japanese Patent Laid Open No. Sho 62-27325 etc., these processes have extremely less production efficiency because of forming from a water-saturated solution of calcium hydroxide (A solubility of Ca(OH).sub.2 is 0.185 g per 100 g of water at 0.degree. C.) and hence are extremely inadequate as processes on a industrial scale. And in Japanese Patent Laid Open No. Sho 58-36924, a process for growing particles by the carbonation within a range of 1-5 in molar ratio of aragonite calcium carbonate to calcium hydroxide is disclosed. However, a particles size of aragonite calcium carbonate prepared in the manner as stated above has at most 1.4-4.5 .mu.m in a long diameter and 0.12-0.6 .mu.m in a short diameter, and it is apt to change into almost calcite type when increasing the above molar ratio for larger particle size.
As mentioned above, it is the present state that a process for commercially producing aragonite calcium carbonate with an acicular shape having a large particle size, namely 10-100 .mu.m level in a long diameter, has not yet been established.